Aristides Zdetsis - Academia.edu (original) (raw)

Papers by Aristides Zdetsis

Journal of Chemical Physics, Mar 1, 1984

Journal of Nuclear Materials

We have developed and implemented on several DEC-5000 work stations, a parallel algorithm suitabl... more We have developed and implemented on several DEC-5000 work stations, a parallel algorithm suitable for distributed concurrent processing, using the Parallel Virtual Machine (PVM). We have used the “regular crowd” structure, in which crowd computations are performed through concurrent processes which are identical. Contrary to the ring structure in which succesive time steps are evolved from one processor to the next, crowd conputations are performed in different groups of space steps. As a basis for the Molecular Dynamics (MD) algorithm, we have taken the MD code of Biswas and Hamann, written in Fortran-77.

In the Frontiers of Computational Science, 2005

Le Journal de Physique Colloques, 1981

Physical Review B, 1979

The Bornvon Karman model of lattice dynamics of diamond structure has been extended to include up... more The Bornvon Karman model of lattice dynamics of diamond structure has been extended to include up to 12th-neighbor interactions. Applications to Ge and Si using eighth-neighbor interactions have been carried out, %'e obtained very good fits to the experimental values of phonon dispersion curves and elastic constants. However, in agreement with the conclusions of Herman, a reasonable fit can only be obtained using at least up to fifth-neighbor interactions. The special significance of the fifth neighbor is attributed to its structure. %'ith the fitted force constants, we have calculated the phonon density of states using the high-resolution Gilat-Raubenheimer method. The calculated Debye temperatures and specific heats compare well with the experimental values.

Springer Proceedings in Physics, 1988

HOMO–LUMO and optical gaps of (CdSe)n nanocrystals appear to have controversial magnitudes and si... more HOMO–LUMO and optical gaps of (CdSe)n nanocrystals appear to have controversial magnitudes and size dependence, which we have rationalized.

The Journal of Chemical Physics, 2004

Polyethyleneimine is a very interesting polymer, not only for its extensive use in biological app... more Polyethyleneimine is a very interesting polymer, not only for its extensive use in biological applications, but also for its crystal structure as a double-stranded helix in the anhydrous state. In order to elucidate the electronic bulk properties of the crystalline (or linear) polyethyleneimine built from ethylenediamine molecules in anhydrous conditions, we performed ab initio density functional theory calculations on water-free molecular crystal structures. The resulting polymer is a semiconductor with a small band gap: Eg=0.40 eV.

Chemical Physics, 1979

... of distant the origin 1 in set (z1, X-,, x3) 1 FC matrix parameters neighbors 4 4 2 4 1 4..J3... more ... of distant the origin 1 in set (z1, X-,, x3) 1 FC matrix parameters neighbors 4 4 2 4 1 4..J3 4 (4,4,4) ~( ss pppa ss fi a 2 L.,J~ 12 [5] (0,1,1) 11 o -5 -o ~ ry 3 4,/11 12 [19] ( 4, 4, 4) fi ~ ss 0 0 z VV /1 4 4~J1~ 6 [29] (0.0.2) fiE 1f o OOO ~ o O i 5 4~Ji~ 12 [37] (4,4,4) fA o, 0 6 ~ yp 6 24 [49 ...

arXiv: Mesoscale and Nanoscale Physics, 2016

We study the optical properties of hydrogen passivated silicon, germanium and mixed Ge/Si core/sh... more We study the optical properties of hydrogen passivated silicon, germanium and mixed Ge/Si core/shell quantum dots (QDs) using high accuracy Density Functional Theory (DFT) and time-dependent DFT (TDFT). We employ the hybrid DFT functional of Becke, Lee, Yang and Parr (B3LYP) in combination with good quality basis sets. As we have shown in our previous work, this combination is an accurate and computationally efficient way for such calculations. The mixed quantum dots, as would be expected, are more versatile and offer more possibilities for band gap engineering, with gap values (electronic and optical) between those of the corresponding Si and Ge dots. Our results support the quantum confinement theory for all three types of QDs.

Solid State Communications, Sep 1, 1978

ABSTRACT

Springer eBooks, 1985

In previous work1,2, we have obtained an understanding of the role of hydrogen in amorphous silic... more In previous work1,2, we have obtained an understanding of the role of hydrogen in amorphous silicon (a-Si) performing first principles calculations of the electronic and transport properties of a-Si:H using the coherent potential approximation (CPA).

In contrast to alternant nanographenes (NGRs), in non-alternant NGRs no “sublattice structure” ca... more In contrast to alternant nanographenes (NGRs), in non-alternant NGRs no “sublattice structure” can be defined associated with significant conceptual and computational simplifications. This leads to some fundamental differences between the two. We uncover here the broken electron-hole symmetry in non-alternant NGRs as one fundamental difference closely related to distorted Dirac points (cones) and their diradical open-shell character. We also show by higher level calculations beyond common DFT that the alternant series of peri-acenes (bisanthene, peri-tetracene, peri-pentacene, … etc.), contrary to opposite reports in the literature, have clearly closed singlet ground states, in contrast to their non-alternant isomers based on Stone-Wales (SW) defects. This is experimentally supported by sub-molecularly resolved STM images. The misconceptions in the literature are due to insufficient correlation. For non-alternant NGRs/GNRs with antiaromatic rings the driving force for open-shell sta...

Applied Surface Science, 2016

In most of the realistic ab initio and model calculations which have appeared on the emission of ... more In most of the realistic ab initio and model calculations which have appeared on the emission of light from silicon nanocrystals, the role of surface oxygen has been usually ignored, underestimated or completely ruled out. We investigate theoretically, by density functional theory (DFT/B3LYP) possible modes of oxygen bonding in hydrogen terminated silicon quantum dots using as a representative case of the Si 29 nanocrystal. We have considered Bridge-bonded oxygen (BBO), Doubly-bonded oxygen (DBO), hydroxyl (OH) and Mix of these oxidizing agents. Due to stoichiometry, all comparisons performed are unbiased with respect to composition whereas spatial distribution of oxygen species pointed out drastic change in electronic and cohesive characteristics of nanocrytals. From an overall perspective of this study, it is shown that bridge bonded oxygenated Si nanocrystals accompanied by Mix have higher binding energies and large electronic gap compared to nanocrystals with doubly bonded oxygen atoms. In addition, it is observed that the presence of OH along with BBO, DBO and mixed configurations further lowers electronic gaps and binding energies but trends in same fashion. It is also demonstrated that within same composition, oxidizing constituent, along with their spatial distribution substantially alters binding energy, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gap (up to 1.48 eV) and localization of frontier orbitals.

The Journal of Physical Chemistry A, 2021

Using a simple, but powerful geometrical/topological notion of aromaticity, based on the shell mo... more Using a simple, but powerful geometrical/topological notion of aromaticity, based on the shell model [J. Phys. Chem. C, 2018, 122, 17526] and the bipartite topology, we uncover, on top of the essential equivalence of the fundamental Hückel's and Clar's rules of aromaticity, the significance of empty peripheral rings, which are shown to be linked to zigzag edge states. Such empty rings can be thought of as "inversion symmetry incompatible". Thus, elimination of these rings under the existing symmetry constrains preserves the aromaticity pattern and leads to substantial improvement of the stability and/or sublattice imbalance, resulting in larger electronic bandgaps, and lack of zigzag edge/end states. Using these ideas, we can illustrate that trigonal D3h-symmetric nanographens cannot be graphene-like, because they are either armchair without Dirac points, or zigzag bonded (triangulenes) topologically frustrated with high spin states. This is also true for hexagonal heteroatomic BN or SiC structures, contrary to homoatomic silicene, germanene, etc. Existing paradigms are highly suggestive that such elimination process could occur naturally.

The Journal of Physical Chemistry C, 2018

By revealing and invoking a novel concrete (topological and electronic) shell structure, inherent... more By revealing and invoking a novel concrete (topological and electronic) shell structure, inherent in hexagonal polycyclic aromatic hydrocarbons (PAHs), it is illustrated that the celebrated but largely empirical Huckel and Clar rules for aromaticity and stability can be derived, interrelated, and connected to shell closures, with well-identified stability and aromaticity maxima versus the number of π-electrons, analogous to the ionization energy curves in the periodical table of the elements, versus atomic number. This is because such shell structure is described in terms of topological/geometrical indices, n, l, m l , s, m s , analogous to the corresponding atomic quantum numbers, demanding (2l + 1) × (2s + 1) occupied π-electrons for given l. The frontier orbitals are composed of core and valence (annulene) orbitals in such a way that the benzene "core orbitals" (l = 0) are expanded and combined, "propagating" successively from benzene to the larger layers, like gigantic superbenzenes, distinguished in those with an aromatic central benzene core, odd n, and satisfying the traditional Huckel/Clar rules, and those with a nonaromatic central ring following the opposite trends. All "superbenzenes" are fully aromatic, satisfying the generalized Huckel's law suggested here. These results can be projected and/or expanded to other PAHs of all sizes up to infinite graphene, elucidating, among other things, graphene's sensitivity to edge morphology and size.